1. Field of the Invention
This invention relates to the general field of optical filters and, in particular, to the manufacture of multi-layered thin-film optical structures.
2. Description of the Prior Art
Optical filters are widely used as components of optical systems. In particular, they are employed in various spectroscopic and optical communication systems where information is transmitted within very narrow spectral bands. In order to retrieve the information contained in a particular band, the remaining spectral background (or spectral noise) has to be filtered out. This is achieved generally using optical filters that pass or reject (stop) the input light within a predetermined narrow-band spectral range.
A wide variety of optical filters is known in the art, the most fundamental one being a simple filter fabricated by depositing a thin-film stack of appropriately selected optical material on a suitable substrate. As commonly understood in the art, the terms “thin” and “optically thin” refer to an optical thickness less than or on the order of the wavelength of the light interest. For example, a third-of-a-micron thick layer of titania (Ti2O5) with a refractive index of about 2.1 in the visible portion of the spectrum is considered optically thin for visible light.
The spectral characteristics of multi-layer thin-film filters are determined by the combination of properties of the layered materials (such as refractive index, reflectance, transmittance, absorbance, density, and homogeneity) and their physical configuration (such thickness and the order of the layers in a stack). Narrow-band interference filters typically require symmetrical structures for proper spectral performance. That is, each layer in the structure has a corresponding layer symmetrically disposed within the multi-layer stack. Multiple such stacks may also be combined to form more complex filters.
Conventional techniques for fabricating thin-film optical filters include, for example, vacuum vapor deposition, deposition by electron-beam evaporation (EBE), techniques based on ion-assisted deposition (IAD), reactive ion plating, and ion-beam sputtering. All techniques utilize the same well-established manufacturing sequence. As illustrated in the multi-layer structure 10 of FIG. 1, a thin-film stack 12, sometime consisting of hundreds of layers, is deposited on a substrate 14 made of suitable optical material. The deposition is carried out in a vacuum chamber under well defined and controlled fabrication conditions and in a strict sequential order designed to produce specific filter characteristic, starting with the first layer 1 and sequentially building the stack up to the last layer n.
As is well understood in the art, narrow-band multi-layer filters include a symmetrical structure wherein a spacer layer separates two mirror-image multi-layered components. Thus, as illustrated in its simplest form in FIG. 2, the stack 12 includes a spacer layer 16 that is also formed during the sequential deposition process. All other layers are deposited such that each layer in the bottom half-stack 18 has a corresponding layer symmetrically deposited in the top half-stack 20. That is, layer 1 is intended to be exactly the same as layer n, layer 2 the same as layer n−1, and so on. A second optical substrate 22 may be also deposited or laminated on top of the multi-layer stack 12 to protect and increase the rigidity of the filter.
The sequential layer deposition of optical material is characterized by an inherent worsening of the material micro-structure in the layers (surface roughness, density, and presence of columnar structure within the volume) as the deposition progresses. This progressive deterioration is due in part to the material and the surface quality of the substrate 14 and in part to the conditions of deposition. It is known that deposition of thin films with very smooth surfaces requires that an extremely smooth and polished substrate be used (with a residual root-mean-square roughness of a few Angstrom, at least less than a nanometer and preferably about 1 Ä for ceramics and metals). However, even under such ideal conditions, the residual structural defects and microscopic non-uniformities of each underlying layer propagate through the thickness of each newly deposited layer and grow more pronounced in the upper layers of the stack. This shortcoming is an especially critical problem in the fabrication of narrow-band interference filters because it materially affects the structural symmetry of the filter (even though macroscopic symmetry may be present). Since light scattering due to structural non-uniformities inevitably leads to broadening of the filter's band, in practice this shortcoming has prevented the reliable fabrication of extremely narrow-band filters (i.e., filters with bandwidths on the order of a few Angstrom or less), especially notch filters.
As a result of such structural deficiencies, the layers of each pair of such symmetrically disposed layers in the stack 12 are not structurally identical. Therefore, they do not perform optically in the same way under equal ambient conditions, which worsens the performance of the filter as a whole by broadening its bandwidth and shifting the peak wavelength of the band. This effect is further worsened by the fact that corresponding layers in the stack, because of their microscopic differences, also tend to react differently to ambient stresses, such as temperature and humidity changes after the stack is removed from the fabrication chamber.
Still another problem lies in the fact that, once a conventional filter has been fabricated, it is practically impossible to correct its spectral performance (such as its precise peak wavelength) by accessing and modifying the inner spacer layer 16 of the filter. This deficiency is very important, especially for very narrow-band etalon-type thin-film filters where the cavity provided by the spacer layer 16 determines the specific spectral characteristics of the filter. An error in the thickness of the spacer layer leads to a spectral shift of the peak wavelength of the interference filter. Therefore, the filter cannot be used for the intended purpose and is practically wasted.
These process drawbacks of the prior-art are unavoidable and contribute to the current very high cost of manufacture of narrow-band multi-layer thin-film filters. Therefore, there remains a need for a manufacturing approach that overcomes the problems and limitations described above.